Lu-Hf garnet geochronology of eclogites from the Balma Unit (Pennine Alps): implications for Alpine paleotectonic reconstructions
Lu-Hf garnet geochronology of eclogites from the Balma Unit (Pennine Alps): implications for Alpine paleotectonic reconstructions
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DOI：
10.1007/s00015-008-1292-y
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发表时间：
2008-11
影响因子：
3.1
通讯作者：
D. Herwartz;C. Münker;E. Scherer;T. Nagel;J. Pleuger;N. Froitzheim
中科院分区：
文献类型：
--
作者：
D. Herwartz;C. Münker;E. Scherer;T. Nagel;J. Pleuger;N. Froitzheim
本文对产于本宁阿尔卑斯山罗莎推覆体顶部的蛇绿岩片--巴尔马单元的三个榴辉岩样品进行了P-T演化、地球化学和Lu-Hf年代学研究。这个单位的古地理起源是有争议的（北宾夕法尼亚州与南宾夕法尼亚州）。它被解释为晚白垩世的洋壳，根据ca。榴辉岩中同岩浆锆石核的93 Ma U-Pb SHRIMP年龄微量元素和同位素数据表明，洋中脊（莫尔），而不是板内或OIB设置的榴辉岩的原岩。对典型石榴石的电子探针分析显示出典型的石榴石分带剖面。550-600 °C的估计峰值变质温度很可能没有超过Lu-Hf系统的闭合温度。因此，Lu-Hf年龄最有可能反映石榴石生长在所研究的样品。为了尽量减少夹杂物对年龄测定的影响，石榴石的选择性消化程序，其中锆石和金红石夹杂物不溶解。三个样品的年龄分别为42.3± 0.6Ma（MSWD：0.47）、42±1 Ma（MSWD：3.0）和45.5± 0.3Ma（MSWD：0.33），比迄今为止报道的南奔尼尼克单元的Lu-Hf年龄都要年轻。42.3Ma样品（PIS 1）中变质锆石畴的U-Pb SHRIMP年龄为40.4± 0.7Ma，表明变质锆石的生长晚于石榴子石的生长，这些新的数据对阿尔卑斯山的古地理重建提供了重要的制约。这些岩石的MORB特征，以及它们先前公布的原岩年龄，意味着在晚白垩世海洋扩张仍在发生。这支持了我们的样品的北宾夕法尼亚起源，因为板块构造模型预测白垩纪扩张在北宾夕法尼亚，而不是在南宾夕法尼亚海洋。如果Balma单元确实是北宾夕法尼亚，新的Lu-Hf数据，结合已发表的地质年代学数据，需要两个独立的俯冲带消耗南，北宾夕法尼亚海洋。
Three samples of eclogite from the Balma Unit, an ophiolite sheet on top of the Monte Rosa Nappe in the Pennine Alps, were investigated in terms of their P-T evolution, geochemistry, and Lu-Hf geochronology. The paleogeographic origin of this unit is controversial (North Penninic vs. South Penninic). It has been interpreted as a piece of Late Cretaceous oceanic crust, on the basis of ca. 93 Ma U-Pb SHRIMP ages of synmagmatic zircon cores in an eclogite. Trace element and isotope data suggest a mid ocean ridge (MOR) rather than an intraplate or OIB setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 550–600 °C most likely did not exceed the closure temperature of the Lu-Hf system. Hence, Lu-Hf ages most likely reflect garnet growth in the studied samples. To minimize inclusion effects on age determinations, a selective digestion procedure for garnet was applied, in which zircon and rutile inclusions are not dissolved. The ages obtained for three samples, 42.3±0.6 Ma (MSWD: 0.47), 42±1 Ma (MSWD: 3.0) and 45.5±0.3 Ma (MSWD: 0.33), are younger than all Lu-Hf ages reported so far for South Penninic Units. Metamorphic zircon domains of the 42.3 Ma sample (PIS1) were previously dated by U-Pb SHRIMP at 40.4±0.7 Ma, indicating that the growth of metamorphic zircon post-dated the onset of garnet growth.These new data put important constraints on the paleogeographic reconstruction of the Alps. The MORB character of the rocks, together with their previously published protolith age, imply that oceanic spreading was still taking place in the Late Cretaceous. This supports a North Penninic origin for our samples because plate tectonic models predict Cretaceous spreading in the North Penninic but not in the South Penninic Ocean. If the Balma Unit is indeed North Penninic, the new Lu-Hf data, in combination with published geochronological data, require that two independent subduction zones consumed the South and North Penninic oceans.